1. Field of the Invention
The present invention relates to a radiation image-acquiring apparatus or a radiation image-acquiring method capable of removing or reducing influences of motion variations in a subject (a person, for example) to be examined (an examined subject) in, for example, X-ray CT (Computer Tomography) apparatuses for performing image-acquiring by using radiation such as X-rays.
2. Related Background Art
Conventionally, an X-ray CT apparatus is known, in which an examined subject is exposed to X-rays, an X-ray detector detects X-rays transmitted through or scattered by the examined subject, and a transmission image, a tomographic image or a three-dimensional image of the subject is acquired based on this X-ray detection output (the number of photons of the X-rays) from the detector.
In connection with such an X-ray CT apparatus, a cone-beam CT apparatus (CBCT) has been developed. While an X-ray beam is thinly sliced in a Z direction (this sliced beam is called a fan beam) in an ordinary X-ray CT apparatus, an X-ray beam expanding also in the Z direction (referred to as a cone beam) is used in the cone-beam CT.
Further, in addition to a conventional CT with a single row of ROW, a type of the CBCT corresponding to a so-called third-generation type or an R/R type has been studied. In the third-generation CT, scanning and acquisition of projection data are executed while a set of an X-ray generating source and a detector is rotated around an examined subject.
FIG. 6 illustrates an example of the CBCT apparatus that belongs to the third-generation CT apparatus. In this apparatus, an X-ray detector 2 is also rotated around an examined subject P together with an X-ray generating source 1 about a rotational Z-axis, and scanning of a region of interest is completed in a single rotation.
In an ordinary X-ray CT apparatus, elements of a detector are arranged in line along a channel (CH) direction since sampling is performed in the CH direction. Individual elements are distinguished by their channel numbers, respectively. In contrast thereto, in a CBCT apparatus, elements of a detector are also arranged along the Z direction (the ROW direction). In other words, elements of the detector are two-dimensionally arranged in the form of an orthogonal lattice in the X-ray detector 2 of the CBCT apparatus.
In such a CBCT apparatus, the X-ray detector 2 is comprised of detector elements arranged in two directions of the Z direction (the ROW direction) and the CH direction in the form of a lattice, and X-rays are emitted in a conical form with an expansion also in the Z direction, so that projection data in a plurality of rows can be simultaneously obtained collectively.
When a plurality of sliced portions are simultaneously photographed, the cone angle is a problem. In a region wherein the cone angle is large, X-rays transmitted through a section of an examined subject are likely to have artifacts, and accordingly a reconstruction error is likely to occur. To avoid this problem, the following only needs to be carried out. That is, a distance (Focus-Detector-Distance: FDD) between a focal point of X-rays and a flat panel detector (FPD) is enlarged, and the cone angle is decreased.
However, as the distance (FDD) increases, an image-acquiring system increases, and it becomes difficult to speedily rotate an X-ray tube and the FPD. Further, it becomes troublesome to place the apparatus in an examination room. It is therefore considered to slowly rotate an examined subject in place of the rotation of the X-ray tube and the FPD to carry out the image-acquiring.
In the event that a human body is rotated, a period of three (3) to five (5) seconds per rotation is considered appropriate. In the image-acquiring for such a long time, however, motions of internal organs due to pulsations of a heart as well as motions of the human body become problems. Especially, in the image-acquiring of a lung area, motions of blood vessels in lungs are outstanding due to pulsations of the heart. As illustrated in FIG. 7, in a period between an R wave and a T wave of an electrocardiogram, the size of a heart abruptly changes, and a large pressure is applied to an aorta, so that the location of the blood vessel is drastically displaced. Such a drastic displacement of the blood vessel not only lowers the resolution of a reconstructed image, but also brings forth artifact, resulting in troubles to diagnosis of a disease.
Japanese Patent Application Laid-Open No. 2000-51208 (JPLO-2000-51208) discloses an X-ray CT apparatus capable of achieving both reduction of the radiation dose of X-ray exposure and prevention of degradation of an image in an electrocardiogram synchronous scanning. This apparatus includes a high-voltage generator for applying a high voltage to an X-ray tube to emit X-rays from the X-ray tube, an X-ray detector for detecting X-rays coming from the X-ray tube through an examined subject, a tomographic-image reconstructing processor for reconstructing a tomographic image based on projection data detected by the detector, an electrocardiograph for measuring an electrocardiogram of the examined subject, and a system controller for controlling the high-voltage generator based on the electrocardiogram such that generation of X-rays can be stopped in a predetermined time within a pulsation cycle of the examined subject and X-rays can be generated in a time other than the predetermined period.
In the apparatus of the Japanese Patent Application Laid-Open No. 2000-51208, a period of the pulsation is morphologically classified into a constriction time, a relaxation time and an equivalent relaxation time, and data is acquired by exposure of X-rays performed in the equivalent relaxation time in which morphological variation is small. It is here assumed that a scanning rate is 0.75 second per rotation, and acquisition of data is executed in a half scanning. On the assumption that a fan angle is 50 degrees, a relation of 0.75(180+50)/360=0.48 (second) holds. In other words, when the equivalent relaxation time is equal to or less than 0.48 second, the half-scanning acquisition of data in the equivalent relaxation time is completed in one rotation.
The length of the equivalent relaxation time will be examined. In the Japanese Patent Application Laid-Open No. 2000-51208, a pulsation period is assumed to be about 1 (second), and it can be seen from FIG. 7 that the equivalent relaxation time can occupy over about 60 percent of one period of the pulsation. Accordingly the equivalent relaxation time lasts about 0.6 second. In short, an invention of the Japanese Patent Application Laid-Open No. 2000-51208 is accomplished on the above-discussed assumption, and a patient is dosed with a β blocker and requested to keep quiet for about a hour to achieve the number of pulsations of 60 times per second.
As discussed above, however, when a stand-up CBCT using a large-sized FPD is considered, a period of over three to five seconds per rotation is needed to rotate the examined subject. Therefore, the invention disclosed in the Japanese Patent Application Laid-Open No. 2000-51208 cannot be applied to a CBCT of that type.